The present invention relates to a piezoelectric adjusting mechanism for positioning a cutting element of a tool according to the preamble of claim 1 and to tools with such a piezoelectric adjusting mechanism according to the preambles of claims 6 and 9.
Piezoelectric crystals are known to have high inherent stiffness, are commercially available and, moreover, are extremely robust. They are therefore used in electromechanical systems for adjustment of the tool cutting edges. A tool with such a piezoelectric system for adjusting the tool cutting edge does not need any sensitive mechanical miniature components such as small setscrews, which require particular manual dexterity of the person who adjusts the tool. Instead, the adjustment can be achieved by means of a simple digital keyboard.
As examples, German Unexamined Application DE 3509240 describes a piezoelectric adjusting mechanism according to the preamble of claim 1 and a tool with such an adjusting mechanism according to the preamble of claim 6. DE 3509240 relates in particular to a drive device which contains at least one piezoelectric crystal. This piezoelectric crystal is braced on one side against an abutment provided on a machine tool. On the other side of the piezoelectric crystal, a tool cutting edge is fastened by means of a retaining mount. When an electric voltage is applied to the piezoelectric crystal, a change in distance between the tool cutting edge and the abutment is imposed by the electrostriction of the crystal, or in other words its elastic deformation in the electric field created by the electric voltage.
In addition, German Unexamined Application DE 4401496 discloses a device for positioning the tool cutting edge according to the preamble of claim 9, for high-precision machining of circular, noncircular and/or noncylindrical inside and/or outside contours, wherein either a tool executes a rotary movement against a workpiece or the workpiece does so against the tool. In this conventional device for positioning the tool cutting edge there is used a piezotranslator, which generates a force in axial direction of a tool shank, this force being transformed via a linkage mechanism disposed between the piezotranslator and the tool cutting edge into a force that acts radially on the tool cutting edge. In this way radial positioning of the tool cutting edge relative to a tool shank is achieved.
When tools with such piezoelectric adjusting mechanisms are used to make cylindrical inside or outside contours, however, it is necessary that the electric field acting on the piezocrystal/piezotranslator be kept constant during operation of the tool, in order thereby to ensure that, once the adjustment of the tool cutting edge has been made, it is maintained throughout the process of cutting with the tool. For this purpose the piezocrystal/piezotranslator must be supplied continuously with electric voltage during operation of the tool. Whereas voltage can be supplied relatively simply to stationary tools, such as a lathe tool, via a plug connection, for example, the voltage supply for tools designed for rotary operation can lead to problems or, because of the fact that cable-less voltage infeed would be necessary for the purpose, can lead to elevated manufacturing costs.
Furthermore, if the adjustment of the tool cutting edge must be kept constant during operation of the tool by continuous injection of voltage into the piezocrystal/piezotranslator, the risk exists that the electric field which acts on the piezocrystal/piezotranslator and which is actually supposed to be constant will be inadvertently changed under the influence of external magnetic or electric interfering fields, which in turn would cause a change in the adjustment of the tool cutting edge. Especially in the case of tools designed for rotary operation, such as boring tools, this risk can be present because of the fact that the piezocrystal/piezotranslator itself is rotating, as is therefore the electric field acting on the piezocrystal/piezotranslator.
The object of the present invention is therefore to provide a piezoelectric adjusting mechanism for tool cutting elements which is capable, without an external voltage supply and without great technical complexity, of reliably maintaining a specified adjustment of the tool cutting elements during operation of the tool.
This object is achieved by the piezoelectric adjusting mechanism according to the features of claim 1. The inventive piezoelectric adjusting mechanism is differentiated from conventional piezoelectric adjusting mechanisms by a retaining mechanism, which ensures that the (first) piezoelectric positioning element, such as a piezocrystal in the form of a round rod, is stabilized by mechanical means in a specified deformation condition. Because of this retaining mechanism, it is ensured that the piezoelectric positioning element cannot revert from its specified deformation condition achieved by the voltage supply back to its initial condition after an interruption of the voltage supply between an external voltage source and the piezoelectric positioning element. Since the specified deformation condition of the piezoelectric positioning element achieved by the voltage supply determines the adjustment of the tool cutting edge by virtue of the connection between the piezoelectric positioning element and the tool cutting edge, a specified adjustment of the tool cutting edge can be achieved by the retaining mechanism independently of a voltage supply and thus regardless of external interfering influences.
In a preferred embodiment, the retaining mechanism comprises a clamping device that substantially completely encloses the piezoelectric positioning element, such as a conventional collar band, whose clamping force is so great that the piezoelectric positioning element is maintained in the specified deformation condition, or in other words remains stabilized and quasi xe2x80x9cfrozenxe2x80x9d, as well as a release device, such as a (second) piezoelectric positioning element, in order to permit release of the clamping device and thus elimination of the clamping force exerted by the clamping device on the piezoelectric positioning element. In this embodiment, the positioning elements can advantageously be energized independently of one another electrically, or in other words by means of a uniform open-loop or closed-loop control device, namely one that is electrically operated. This permits a particularly precise adjustment of the tool cutting element.
A particularly simple embodiment of the inventive adjusting mechanism is achieved when the cutting element is securely connected to the first positioning element, by means of a retaining mount or directly. The use of a retaining mount provides the special benefit that the clamping device can grip the entire volume of the piezoelectric positioning element (piezo positioning actor).
The inventive adjusting mechanism is generally applicable for any tool or any form of machining by material removal, such as a lathe, boring, milling or reaming tool, which has a tool shank and at least one cutting element mounted adjustably thereon.
By virtue of a positioning range of up to 100 xcexcm with a resolution of better than 0.1 xcexcm and a loadability of more than 500 N, however, the inventive adjusting mechanism is used in particular in precision boring tools or reamers, in which extremely precise adjustment of the cutting element or elements, such as PCD or metal carbide cutting inserts, is needed in view of the high dimensional, shape and positional accuracy, surface quality and surface texture to be achieved for the bore to be machined.
The inventive precision boring tool preferably has two cutting elements disposed diametrally in recesses on the outside circumference of the tool shank. In this case it is particularly advantageous when the (first) piezoelectric positioning element is placed between the two diametrally disposed cutting elements in such a way that it can be connected directly to both cutting elements. Provided the piezoelectric positioning element can be disposed in central position in radial direction relative to the axis of rotation of the precision boring tool, which can be achieved by simple structural features on the tool shank and/or on the piezoelectric positioning element, synchronous adjustment of both cutting elements in radial direction of the tool shank can therefore be achieved by application of an electric voltage to the piezoelectric positioning element and the resulting elastic deformation of the piezoelectric positioning element. In this way the desired adjustment of the cutting elements can be reliably achieved durably by means, for example, of the collar band cited hereinabove, which embraces the (first) piezoelectric positioning element and clamps it securely such that relaxation of its elastic deformation is not possible.
If a conical bore, for example, is to be created by means of the precision boring tool, the inventive adjusting mechanism can be additionally provided with a further (third) piezoelectric positioning element, which in relation to the tool shank is at a distance axially from the first piezoelectric positioning element and can be electrically energized independently thereof. Hereby tapering of the cutting elements can be achieved in simple manner.
The voltage supply of the piezoelectric positioning elements is achieved in simple manner by providing, on the tool shank, a common terminal element which permits independent electrical energization of the positioning elements, or in other words independent infeed of the adjusting voltage and of the release voltage. In order to avoid unbalance during rotary operation of the precision boring tool, the common terminal element preferably has rotationally symmetric structure. For this purpose the common terminal element can comprise, for example, a plurality of slip rings, which are appropriately associated with the respective positioning elements, one of the slip rings being able to function as the common terminal connection. By this configuration, electrical energization of the piezoelectric positioning elements and thereby adjustment of the cutting elements during operation of the tool is made possible even in the case of a precision boring tool designed for rotary operation.
A particularly advantageous embodiment of the voltage supply is achieved when, according to claim 18, a connector part via which energization of the at least one piezoelectric positioning element and of the retaining mechanism takes place is provided on the tool. The connector part can be provided at any appropriate position on the tool shank, although it is preferably formed at a position which is shielded as well as possible from chips, coolant, lubricant or temperature effect during use of the tool.
A particularly advantageous position of the connector part, such as described in claim 19, is achieved, for example, when the tool is equipped with a hollow taper shank, which can be connected via an HSK coupling to a tool carrier part, such as a tool system module or a spindle. Such HSK couplings are becoming used increasingly more frequently as junction points or disconnection points in the embodiment, when it is necessary to group together tools with particularly high stiffness in modular relationship or to couple them to a machine spindle. Examples of such HSK chucking systems are disclosed in German Patents 3807140 C2 or DE 4220873 A1, to the content of which reference is expressly made here.
The relatively large-area pair of radial end faces which is provided in such couplings and via which the parts to be coupled are braced against one another is exploited advantageously in the improvement of claim 19 in order to shield the connector part from external influences. During operation of the tool the connector part either can be deactivated, in which case the connector part is merely sealed on all sides by the pair of radial annular faces, or, via a connector-part mating piece in the tool-carrier part, can be connected continuously to corresponding control leads in the tool-carrier part, such as in a spindle, in which case the rotationally symmetric terminal element is integrated into the tool-carrier part in order to transmit the electric control signals during tool operation.